Why is it so hard for my in-laws to treat the kids fairly?

Well here's the situation. I have two kids from a previous marriage and my husband has one. My family has no problem accepting my husband's daughter into our family. They buy for all three kids the same and always make sure that they are all treated equally.

Different story when it comes to my in-laws. They pretend like they're going to be fair at holidays and spend the same on all the kids, but always end up "finding" more gifts for my step-daughter.

This hurts my kids' feeling and they want to know why she always gets more than them. This is causing "BIG" problems in my marriage. I know that they are my husbands parents but no good mother stands around while her kids are being treated wrongly. What should I do?